UNCOVERING THE IMMUNOLOGICAL INTERACTIONS BETWEEN RAINBOW TROUT (Oncorhynchus mykiss) AND THE COLDWATER PATHOGEN, Flavobacterium psychrophilum
Like any form of agriculture, aquaculture places fish in conditions deviating significantly from their natural state, making the animals easy targets for opportunistic pathogens. One such pathogen causing major problems in global rainbow trout aquaculture is Flavobacterium psychrophilum, the causative agent of Bacterial Coldwater Disease (BCWD). Outbreaks of BCWD tend to occur at temperatures between 8-12oC with losses reported to be as high as 30%. Despite the impact on stocks worldwide, there is very little known about the pathogenesis of this bacterium. Previous studies have alluded to the possibility of the F. pschrophilum life cycle being intricately involved with spleen and head kidney macrophages. To further study this, RTS-11, a rainbow trout monocyte/macrophage-like cell line was used and infected with F. psychrophilum. Immune proteins, bacterial location and effects on cell morphology/viability were observed throughout infection. Gaining a deeper understanding of the pathogenesis of F. psychrophilum may aid in various preventative measures for this troublesome disease.
Despite the lack of information regarding the pathogenesis of F.psychrophilum, resistance to the disease has been observed to be heritable while not being adversely correlated with the size of the fish. Because there is currently no effective vaccine for BCWD, this knowledge means that selective breeding may be an attainable method of control against the disease. After observing resistance and susceptibility to F. psychrophilum infection in full sibling families of rainbow (results displayed in Table 1), two resistant and two susceptible families were selected so that the functional immune responses of these groups could be analyzed. Respiratory burst activity of head kidney leukocytes was observed to dramatically decrease after seven days of infection which was also the time when fish succumbed to the disease. Understanding the functional immune differences in fish that are capable of mounting an effective immune response versus those that are incapable could provide essential information for the development of a much needed, effective vaccine.